1. Field of the Invention
The present invention relates to an optical resonator and a laser oscillator including a solid-state laser medium and a laser resonator and is suitably applied to a microchip laser.
2. Description of the Related Art
Recently, an excitation method for solid-state lasers using a semiconductor laser has been developed, and the size of the solid-state lasers has been reduced and the efficiency and life thereof have been increased compared to the case where a discharge tube is used for excitation.
In particular, microchip lasers having a thin solid-state laser medium have been developed as small laser oscillators for achieving low-order, longitudinal-mode oscillation and obtaining beams of a desirable shape (refer to, for example, “The Review of Laser Engineering”, Vol. 26 (1998), p. 848 written by Takunori Taira).
Generally, laser resonators included in the microchip lasers are plane-parallel laser resonators obtained by grinding the solid-state laser medium to reduce the thickness thereof and forming mirror surfaces on both sides of the solid-state laser medium.
However, the plane-parallel laser resonators are basically unstable, and the degree of parallelism between the mirror surfaces, that is, the degree of parallelism between the surfaces of the crystal of the solid-state laser medium, must be as high as possible in order to achieve the laser oscillation.
In addition, in the plane-parallel laser resonators, the laser oscillation is achieved using a thermal lens effect in which the reflective index of the solid-state laser medium varies in correspondence with the temperature distribution caused by focusing excitation light on the solid-state laser medium.
However, even if the thermal lens effect is enhanced by increasing the excitation intensity and the laser beam is converged so that the degree of parallelism in the laser resonator can be increased enough to achieve the laser oscillation, the beam shape is distorted when the mirror surfaces of the resonator are even slightly inclined.
In addition, if the excitation intensity is low, the thermal lens effect reduces and a sufficient degree of parallelism cannot be obtained. Therefore, stable laser oscillation cannot be achieved.
Accordingly, in order to achieve stable laser oscillation, laser resonators having a concave mirror may be used. In such a laser resonator, the concave mirror is provided as one of the mirrors forming the laser resonator.
For example, a first main surface of the crystal of a thin solid-state laser medium serves as a flat mirror and a second main surface transmits resonating light. Thus, the light passes through the second main surface and resonates on the concave mirror.
However, when the concave mirror is simply disposed by itself, the central region of the concave mirror is farther away from the flat mirror compared to the peripheral region thereof and the number of components for positioning the concave mirror and the solid-state laser medium increases. Therefore, the cavity length increases accordingly.
Preferably, the concave mirror and the solid-state laser medium can be aligned easily.